The present invention relates to a method of preparing a high-purity metal and a rotary cooling member for use in an apparatus therefor. More particularly, the invention relates to a method of purifying a metal, such as aluminum, silicon, magnesium, lead or zinc, containing eutectic impurities utilizing segregation solidification to prepare a high-purity metal having a lower content of eutectic impurities than the original metal, and to a rotary cooling member for use in an apparatus for the method.
The term "eutectic impurities" as used herein and in the appended claims refers to impurities which are eutectic with the metal to be purified.
For example, a method is known of purifying to a higher purity aluminum containing impurities forming eutectices with aluminum, by utilizing the principle of segregation solidification. This method comprises melting the aluminum to be purified, placing the molten aluminum into a crucible, maintaining the melt at a temperature higher than the solidification temperature thereof at all times, immersing into the melt a hollow rotary cooling member in the form of a downwardly tapered cylinder, and rotating the cooling member while introducing a cooling fluid into the cooling member to maintain the surface of the member at a temperature not higher than the solidification temperature so that the impurities released in the vicinity of the solid-liquid interface when aluminum crystallizes out on the outer peripheral surface of the cooling member are dispersed through and mixed with the entire liquid phase to reduce the thickness of the layer of concentrated impurities in the vicinity of the solid-liquid interface, consequently causing aluminum to crystallize out on the peripheral surface with a higher purity while giving an increased temperature gradient to the layer of concentrated impurities in the liquid phase (Examined Japanese Patent Application SHO No. 61-3385). The principle of this method is described also in U.S. Pat. No. 4,469,512. In order to decrease the thickness of the concentrated impurity layer in the liquid phase in the vicinity of the solid-liquid interface and thereby afford an increased temperature gradient for an improved purifying efficiency, the speed of the cooling member relative to the molten aluminum must be increased as one of the requirements therefor. However, there is a limitation to the increase in the relative speed since the rotation of the cooling member causes the molten aluminum to flow in the same direction as the direction of rotation of the cooling member to produce a whirl, posing a limitation on the improvement in the purification efficiency. Moreover, an increase in the speed of rotation of the cooling member produces a greater centrifugal force, which presents difficulty in the deposition of crystals of high-purity aluminum on the peripheral surface of the cooling member to result in lower productivity. The increased speed makes the melt surface wavy and permits air to be incorporated into the molten aluminum and to react with the aluminum, forming a large quantity of Al.sub.2 O.sub.3 as scum and entailing a need for skimming. The scum scatters about, adhering to the inner surface of the crucible and causing trouble to the operation. The large quantity of scum is likely to impair the purification efficiency. The molten aluminum surface is liable to wave markedly when the formation of an aluminum block starts on the peripheral surface of the cooling member.
To decrease the speed of the whirling flow of molten aluminum, therefore, it has been proposed to attach speed reducing baffles to the inner periphery of the crucible as arranged circumferentially thereof at a predetermined spacing (see Examined Japanese Utility Model Application SHO 61-38912). This arrangement makes it possible to give an increased speed to the cooling member relative to the molten aluminum without very greatly increasing the rotational speed of the cooling member, whereby an improved purification efficiency can be achieved. Nevertheless, the presence of the baffles locally varies the flow velocity of the molten aluminum and consequently produces a flow of molten aluminum having an upward velocity component in the vicinity of an upper part of the portion of the cooling body which is present below the melt surface. As a result, the surface of the molten aluminum becomes more markedly waved to further enhance the adverse effect due to the waving.
These problems are encountered similarly also with the above-mentioned metals other than aluminum.